1. Field of the Invention
This invention relates to mooring systems for ships, barges and other floating structures which are referred to herein as "vessels". The invention relates particularly to mooring systems for vessels which receive or discharge fluid cargo while they are moored, for example, barges and ships which receive or discharge liquid or gas, such as LNG (liquified natural gas), LPG (liquified petroleum gas), oil or fuel gas.
An object of this invention is to provide mooring systems of the above type which are free of certain of the deficiencies of the prior art. A further object is to provide such systems which can be used for barges which are stationed at a particular location for accumulating or storing LNG. These and other objects will be in part obvious and in part pointed out below.
2. Description of the Prior Art
The increased importance of LNG has resulted in the development of systems for mooring vessels such as barges where the gas is received and liquified, and such installations which provide regasification facilities. A publication of Moss Rosenberg Verft A/S entitled "The Moss Rosenberg Marine LNG System", September 1978, discloses a marine LNG system with a mooring arrangement for barges having LNG storage tanks, liquefaction facilities for gas, and facilities for transferring the gas to the barge and the LNG cargo to tankers. A hinged yoke on the barge is attached to an upright leg or "tension leg" which is anchored to the sea bed and which has a large buoyancy container which tends to urge the tension leg toward an upright position. The tension leg is hinged at its bottom end to the anchoring means and at its top end to the yoke. Transfer lines for fluid cargo extend down the tension leg and have movable connecting structures or couplings which permit the lines extending from the barge to have free movement with respect to stationary lines extending from the shore or elsewhere to the anchoring position. Those connections or couplings are below the surface of the water, and that prevents ready inspection and servicing. That mooring system may also be subject to damage by collision, for example, by a tanker moving to or from a position alongside the barge.
A similar mooring system is disclosed in "OCEAN INDUSTRY", November 1978, and is identified as incorporating a special triaxial swivel universal joint in the piping of the cargo transfer lines. That system also has an underwater upright leg which is connected by a universal joint at its lower end to anchoring means on the sea bed. In each of those systems, the upright leg moves about its lower end, and the barge moored to it can swing generally around the mooring point formed by the pivot provided by the attachment of the bottom end of the upright leg to the anchor.
With both of the systems just discussed, the general approach is to provide a riser projecting generally upwardly from the anchor and pivoted at its bottom end, and to a mooring yoke. The pivoting action at the sea bottom can create problems because of the rotatable connections in the line or lines for transferring liquid or gas to or from the moored vessel. Also, the upright leg is often completely submerged so that its location cannot always be determined by observation from a vessel approaching or passing the barge which is moored. It is a further object of the present invention to provide mooring systems which overcome difficulties which are present with prior mooring systems such as those discussed above.